Muscles of Mastication
Mastication is the process of grinding and chewing food into smaller pieces in the oral cavity turning it into a food bolus. This mass can then be swallowed with ease and further digested as it passes along the alimentary canal. Apart from the teeth and the tongue, certain muscles known as the masticatory muscles partake in specific movements of the temporomandibular joint (TMJ) during this process to allow the initial stages of digestion to occur. All muscles of mastication are supplied by branches of the mandibular nerve (V3), a branch of the trigeminal nerve.
This is the main masticatory muscle which causes mandibular elevation and protrusion and assists the other muscles to move the mandible laterally. The masseter originates from the inferior border of the anterior two thirds of the zygomatic arch and promptly terminates at the mandibular angle and the inferior and lateral borders of the mandibular ramus. It is innervated by the masseteric branch of the mandibular nerve.
This muscle elevates and retrudes the mandible while also assisting the lateral pterygoid muscle to move it laterally. It originates from the inferior temporal line which lies within the temporal fossa. The fibers converge in a fan like shape from all around the fossa and insert into the coronoid process of the mandible. The mandibular nerve gives off the superficial and deep temporal branches which innervate both areas of the temporal muscle respectively.
Medial Pterygoid Muscle
The medial pterygoid elevates, protrudes and causes the lateral excursion of the mandible. The muscle has a deep and superficial head. They arise from the medial surface of the pterygoid plate and both the maxillary tuberosity and palatine process respectively. Insertion occurs upon the pterygoid tubercles that can be found on the medial surface of the ramus and the mandibular angle. The mandibular nerve innervates the medial pterygoid via the medial pterygoid branch.
Lateral Pterygoid Muscle
This lateral ptyergoid muscle muscle depresses and protrudes the mandible and moves it laterally. It consists of a superior and inferior head. The superior head originates from the infratemporal crest on the greater wing of the sphenoid bone and inserts into the pterygoid fovea on the condylar neck of the mandible.
In contrast, the inferior head runs from the lateral surface of the lateral pterygoid plate to the condylar process of mandible. The lateral pterygoid muscle is supplied by the lateral pterygoid nerve branching off the mandibular nerve.
Aside from direct trauma, the most common pathological ailment affecting the muscles of mastication is a condition known as trismus (or lockjaw). It is defined as a prolonged tonic spasm of the jaw muscles. The affected patients find it very difficult and in some cases impossible to open the mouth.
Differential diagnosis is an absolute necessity and further investigation is usually required in order to pinpoint the exact cause of the trismus. The muscles are usually not the primary cause, however they can be included in the problem due to the spread of dental infection into the fascias that surround them. Treatment includes eliminating the main cause of infection.